U.S. patent number 7,090,744 [Application Number 10/128,973] was granted by the patent office on 2006-08-15 for process for making composition for conversion to lyocell fiber from sawdust.
This patent grant is currently assigned to Weyerhaeuser Company. Invention is credited to Mengkui Luo, W. Harvey Persinger, Jr., James E. Sealey, II, Brian Wester.
United States Patent |
7,090,744 |
Sealey, II , et al. |
August 15, 2006 |
**Please see images for:
( Certificate of Correction ) ** |
Process for making composition for conversion to lyocell fiber from
sawdust
Abstract
A process for making a composition for conversion to lyocell
fiber where the process comprises pulping a raw material in a
digester to provide an alkaline pulp, wherein the raw material
comprises sawdust in an amount greater than 0 % up to 100 %; and
contacting the alkaline pulp comprising cellulose and at least
about 7 % hemicellulose under alkaline conditions with an amount of
an oxidant sufficient to reduce the average degree of
polymerization of the cellulose to within the range of from about
200 to about 1100, without substantially reducing the hemicellulose
content of the pulp or substantially increasing the copper
number.
Inventors: |
Sealey, II; James E. (Federal
Way, WA), Persinger, Jr.; W. Harvey (Enumclaw, WA), Luo;
Mengkui (Tacoma, WA), Wester; Brian (Sumner, WA) |
Assignee: |
Weyerhaeuser Company (Federal
Way, WA)
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Family
ID: |
25286927 |
Appl.
No.: |
10/128,973 |
Filed: |
April 23, 2002 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030025251 A1 |
Feb 6, 2003 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09842274 |
Apr 24, 2001 |
6605350 |
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09574538 |
May 18, 2000 |
6331354 |
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09256197 |
Feb 24, 1999 |
6210801 |
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09185423 |
Nov 3, 1998 |
6306334 |
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09039737 |
Mar 16, 1998 |
6235392 |
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08916652 |
Aug 22, 1997 |
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60023909 |
Aug 23, 1996 |
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60024462 |
Aug 23, 1996 |
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Current U.S.
Class: |
162/91; 162/67;
162/90; 162/78; 162/65 |
Current CPC
Class: |
D01F
2/00 (20130101); D01D 5/18 (20130101); D21C
9/004 (20130101); D01D 5/098 (20130101); C08B
1/00 (20130101); D21C 9/10 (20130101); D21C
3/02 (20130101); Y10T 428/2933 (20150115); Y10T
428/2965 (20150115); Y10T 428/29 (20150115); Y10T
428/2913 (20150115) |
Current International
Class: |
D21C
3/02 (20060101); D01C 1/00 (20060101); D21C
9/00 (20060101) |
Field of
Search: |
;162/9,90,79,65-67,71,78,87-91,142,146,150,157.1,157.6,157.7
;264/203,233,211.15,211.14,5,187 ;428/395,357,364,375,393
;8/116.1,125 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2071185 |
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Dec 1992 |
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CA |
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0 157 179 |
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Sep 1985 |
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EP |
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0 785 304 |
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Jul 1997 |
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EP |
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2735794 |
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Dec 1996 |
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FR |
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1 253 234 |
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Nov 1971 |
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GB |
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WO 98/22642 |
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May 1998 |
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JP |
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WO 95/35399 |
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Dec 1995 |
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WO |
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WO 96/12063 |
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Apr 1996 |
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WO |
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WO 96/25552 |
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Aug 1996 |
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WO |
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WO 96/27700 |
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Sep 1996 |
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WO |
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WO 97/23666 |
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Dec 1996 |
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WO |
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WO 97/15713 |
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May 1997 |
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WO |
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WO 97/30196 |
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Aug 1997 |
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WO |
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WO 98/02662 |
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Jan 1998 |
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WO |
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WO 98/51855 |
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Nov 1998 |
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WO |
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WO 99/16960 |
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Apr 1999 |
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WO |
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WO 99/47733 |
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Sep 1999 |
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WO |
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of Chemical Technology, 4th ed., John Wiley & Sons, vol. 10,
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696-726. cited by other.
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Primary Examiner: Hug; Eric
Attorney, Agent or Firm: Christensen O'Connor Johnson
Kindness
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a divisional of application Ser. No.
09/842,274, filed Apr. 24, 2001, now U.S. Pat. No. 6.605.350, which
is a continuation-in-part of application Ser. No. 09/574,538, filed
May 18, 2000, now U.S. Pat. No. 6,331,354, which is a
continuation-in-part of application Ser. No. 09/256,197, filed Feb.
24, 1999, now U.S. Pat. No. 6,210,801, which is a
continuation-in-part of application Ser. No. 09/185,423, filed Nov.
3, 1998, now U.S. Pat. No. 6,306,334, which is a
continuation-in-part of application Ser. No. 09/039,737, filed Mar.
16, 1998, now U.S. Pat. No. 6,235,392, which is a
continuation-in-part of application Ser. No. 08/916,652, filed Aug.
22, 1997, now abandoned, which claims priority from Provisional
Application Ser. Nos. 60/023,909 and 60/024,462, both filed Aug.
23, 1996.
Claims
The invention claimed is:
1. A process for making a composition for conversion to lyocell
fiber, said process comprising: pulping a raw material in a
digester to provide an alkaline pulp, wherein the raw material
comprises sawdust in an amount greater than 25% up to 100%; and
contacting the alkaline pulp comprising cellulose and at least
about 7% hemicellulose under alkaline conditions with an amount of
an oxidant sufficient to reduce the average degree of
polymerization of the cellulose to within the range of from about
200 to about 1100, without substantially reducing the hemicellulose
content of the pulp or substantially increasing the copper
number.
2. The process of claim 1, wherein said oxidant comprises at least
one member of the group consisting of a chemical with a peroxide
group, hydrogen peroxide, oxygen, chlorine dioxide, ozone and
combinations thereof.
3. The process of claim 1, wherein the reduction in the average
degree of polymerization of the cellulose occurs in the presence of
a ratio of magnesium to transition metals of less than about
50%.
4. The process of claim 1, wherein the hemicellulose content of the
pulp is reduced less than about 50%.
5. The process of claim 4, wherein the hemicellulose content of the
pulp is reduced less than about 15%.
6. The process of claim 5, wherein the hemicellulose content of the
pulp is reduced less than about 5%.
7. The process of claim 1, wherein the copper number increases less
than about 50%.
8. The process of claim 7, wherein the copper number increases less
than about 25%.
9. The process of claim 1, wherein the contacting step further
comprises contacting the pulp with an alkali source selected from
the group consisting of sodium hydroxide, oxidized white liquor,
and unoxidized white liquor.
10. The process of claim 9, wherein the alkaline pulp and oxidant
are contacted at a pH greater than about 8.
11. The process of claim 1, further comprising a step for
dissolving the composition in a solvent, after the step of
contacting the alkaline pulp with an oxidant, wherein the
composition is readily dissolved in NMMO monohydrate within 5
minutes for forming into a spinnable dope.
12. The process of claim 1, wherein the digester is a continuous
digester.
13. The process of claim 12, wherein the digester is an M&D or
Pandia-type digester.
14. The process of claim 1, wherein the sawdust comprises less than
or about 50% of fibers retained on a one-quarter inch screen by
TAPPI standard 233.
15. The process of claim 14, wherein the sawdust comprises less
than about 30% of fibers retained on a one-quarter inch screen by
TAPPI standard 233.
16. The process of cLaim 15, wherein the sawdust comprises less
than about 15% of fibers retained on a one-quarter inch screen by
TAPPI standard 233.
17. The process of claim 1, wherein said raw material comprises
about 50% up to 100% sawdust.
Description
FIELD OF THE INVENTION
The present invention is directed to a composition useful for
making lyocell fibers, where the composition is made from a sawdust
pulp.
BACKGROUND OF THE INVENTION
Cellulose is a polymer of D-glucose and is a structural component
of plant cell walls. Cellulose is especially abundant in tree
trunks from which it is extracted, converted into pulp, and
thereafter utilized to manufacture a variety of products. Rayon is
the name given to a fibrous form of regenerated cellulose that is
extensively used in the textile industry to manufacture articles of
clothing. For over a century strong fibers of rayon have been
produced by the viscose and cuprammonium processes. The latter
process was first patented in 1890 and the viscose process two
years later. In the viscose process cellulose is first steeped in a
mercerizing strength caustic soda solution to form an alkali
cellulose. This is reacted with carbon disulfide to form cellulose
xanthate which is then dissolved in dilute caustic soda solution.
After filtration and deaeration the xanthate solution is extruded
from submerged spinnerets into a regenerating bath of sulfuric
acid, sodium sulfate, zinc sulfate, and glucose to form continuous
filaments. The resulting so-called viscose rayon is presently used
in textiles and was formerly widely used for reinforcing rubber
articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammonia copper oxide.
This property forms the basis for production of cuprammonium rayon.
The cellulose solution is forced through submerged spinnerets into
a solution of 5% caustic soda or dilute sulfuric acid to form the
fibers, which are then decoppered and washed. Cuprammonium rayon is
available in fibers of very low deniers and is used almost
exclusively in textiles.
The foregoing processes for preparing rayon both require that the
cellulose be chemically derivatized or complexed in order to render
it soluble and therefore capable of being spun into fibers. In the
viscose process, the cellulose is derivatized, while in the
cuprammonium rayon process, the cellulose is complexed. In either
process, the derivatized or complexed cellulose must be regenerated
and the reagents that were used to solubilize it must be removed.
The derivatization and regeneration steps in the production of
rayon significantly add to the cost of this form of cellulose
fiber. Consequently, in recent years attempts have been made to
identify solvents that are capable of dissolving underivatized
cellulose to form a dope of underivatized cellulose from which
fibers can be spun.
One class of organic solvents useful for dissolving cellulose are
the amine-N oxides, in particular the tertiary amine-N oxides. For
example, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group
of amine oxide materials suitable as solvents. Johnson, in U.S.
Pat. No. 3,447,939, describes the use of anhydrous
N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as
solvents for cellulose and many other natural and synthetic
polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282,
deals with the difficulties of dissolving cellulose in amine oxide
solvents and of achieving higher concentrations of cellulose.
Lyocell is an accepted generic term for a fiber composed of
cellulose precipitated from an organic solution in which no
substitution of hydroxyl groups takes place and no chemical
intermediates are formed. Several manufacturers presently produce
lyocell fibers, principally for use in the textile industry. For
example, Acordis, Ltd. presently manufactures and sells a lyocell
fiber called Tencel.RTM. fiber.
Currently available lyocell fibers are produced from high quality
wood pulps that have been extensively processed to remove
non-cellulose components, especially hemicellulose. These highly
processed pulps are referred to as dissolving grade or high alpha
(or high .alpha.) pulps, where the term alpha (or .alpha.) refers
to the percentage of cellulose. Thus, a high alpha pulp contains a
high percentage of cellulose, and a correspondingly low percentage
of other components, especially hemicellulose. The processing
required to generate a high alpha pulp significantly adds to the
cost of lyocell fibers and products manufactured therefrom.
Typically, the cellulose for these high alpha pulps comes from
softwood, which generally has longer fibers than hardwoods.
Softwoods must be made into wood chips to make them suitable to be
pulped in the digesters. The digesters are normally equipped with a
system for recycle of the black liquor. Recycle provides a
desirable homogenous mixture throughout the digester that leads to
uniform pulping conditions. In order to move the black liquor,
digesters are equipped with screens to keep wood chips from the
pump inlet.
Since conventional Kraft processes stabilize residual
hemicelluloses against further alkaline attack, it is not possible
to obtain acceptable quality dissolving pulps, i.e., high alpha
pulps, through subsequent treatment of Kraft pulp in the bleaching
stages. Thus, in order to prepare dissolving type pulps by the
Kraft process, it is necessary to give the raw material an acidic
pretreatment before the alkaline pulping stage. A significant
amount of material primarily hemicellulose, on the order of 10% or
greater of the original wood substance, is solubilized in this acid
phase pretreatment and thus process yields drop. Under the
prehydrolysis conditions, the cellulose is largely resistant to
attack, but the residual hemicelluloses are degraded to a much
shorter chain length and can therefore be removed to a large extent
in the subsequent Kraft cook by a variety of hemicellulose
hydrolysis reactions or by dissolution.
A relatively low copper number, reflective of the relative carbonyl
content of the cellulose, is a desirable property of a pulp that is
to be used to make lyocell fibers because it is generally believed
that a high copper number causes cellulose and solvent degradation,
before, during, and/or after dissolution in an amine oxide solvent.
The degraded solvent can either be disposed of or regenerated;
however, due to its cost it is generally undesirable to dispose of
the solvent. Regeneration of the solvent suffers from the drawback
that the regeneration process involves dangerous, potentially
explosive conditions.
A low transition metal content is a desirable property of a pulp
that is to be used to make lyocell fibers because, for example,
transition metals accelerate the undesirable degradation of
cellulose and NMMO in the lyocell process.
In view of the expense of producing commercial dissolving grade
pulps, it would be desirable to have alternatives to conventional
high alpha dissolving grade pulps as a lyocell raw material. In
addition, pulp manufacturers would like to minimize the capital
investment necessary to produce such types of pulps by utilizing
existing capital plants.
In order to control lyocell fiber properties, lyocell manufacturers
utilize dopes that comprise a blend of different pulps having
different ranges of average degree of polymerization values. In
view of this, there is also a need for pulp manufacturers to
produce pulps having an average degree of polymerization within a
broad band to eliminate the need for blending.
Thus, there is a need for relatively inexpensive, low alpha (e.g.,
high yield) pulps that can be used to make lyocell fibers, for a
process of making the foregoing low alpha pulps using capital
equipment that is currently available to pulp manufacturers, and
for lyocell fibers from the foregoing low alpha pulp. Preferably,
the desired low alpha pulps will have a desirably low copper
number, a desirably low lignin content and a desirably low
transition metal content but broad molecular weight
distribution.
In a prior application having a Ser. No. 09/256,197, the disclosure
of which is herein incorporated by reference, assigned to the
assignee of the present application, various methods of reducing
D.P. values and copper number of a Kraft pulp are described. Such
methods include treating pulp with acid, or an acid substitute, or
a combination of acids and acid substitutes. Other means of
treating the pulp to reduce the average D.P. of cellulose without
substantially reducing the hemicellulose content described in the
prior application include treatment of the pulp with steam, a
combination of ferrous sulfate and hydrogen peroxide, at least one
transition metal and peracetic acid, an alkaline chlorine dioxide
treatment which ends acidic or a sodium hypochlorite treatment
which ends near neutral. Such processes are effective at reducing
the average degree of polymerization without substantially reducing
the hemicellulose content, however, such processes can be expensive
from a capital improvement standpoint if the existing pulp mills in
which such processes are to be used are not configured to allow for
the simple deployment of such processes. In the prior application,
additional steps are described in order to reduce the copper number
of the pulp which has been treated to reduce its average degree of
polymerization without substantially decreasing the hemicellulose
content. The need for this subsequent copper number reducing step
arose because the methods described in the prior application for
reducing the average degree of polymerization for the cellulose
resulted in an increase in the copper number for the resultant
pulp.
In view of environmental concerns, there has been a great interest
in using bleaching agents, which reduce the amount of
chlorocompounds that must be recovered from process streams. In
recent years, the use of oxygen as a delignifying agent has
occurred on a commercial scale. Examples of equipment and apparatus
useful for carrying out an oxygen stage delignification are
described in U.S. Pat. Nos. 4,295,927; 4,295,925; 4,298,426; and
4,295,926.
While the methods described in prior application '197 are effective
at reducing the average D.P. of cellulose without substantially
decreasing the hemicellulose content, a further need still existed
for a process that did not require a separate copper number
reducing step and which was readily adaptable to pulp mills that
include oxygen reactors, multiple alkaline stages and/or alkaline
conditions suitable for substantial D.P. reduction of bleached or
semi-bleached pulp. In a more recent application having Ser. No.
09/574,538, assigned to the assignee of the present application,
the disclosure of which is herein incorporated by reference, the
assignee of the present application discovered how to make a highly
desirable low viscosity pulp from an alkaline pulp by treatment of
the alkaline pulp under conditions of an oxidizing agent in a
medium to high consistency reactor to reduce the D.P. of the
cellulose, without substantially reducing the hemicellulose or
increasing the copper number.
Presently, the forest industry generates vast quantities of sawdust
byproduct in the normal course of day-to-day wood processing. While
some sawdust has found its way into pulping mills that use
digesters made to be used with sawdust, a large proportion of the
total byproduct sawdust remains unused. Even so, the conventional
pulp made from sawdust in M&D or Pandia digesters is considered
unsuitable to be used as a dissolving type pulp. One of the
drawbacks to using sawdust in a digester suited to make high alpha
pulp is that the recycle stream continuously plugs with the sawdust
particulates. The typical sawdust pulp currently being made
therefore does not have a high alpha content or viscosity level
suitable for use as dissolving pulp. The typical sawdust pulp also
contains contaminates (dirt, metal or plastic) that are
unacceptable for dissolving grade pulps.
The need to conserve resources by utilizing sawdust byproducts,
however, presents a unique opportunity. It would be advantageous to
develop a pulp and method using the conventional sawdust digesters
to produce a dissolving type pulp that would be useful for making
lyocell molded bodies from the byproducts of wood processing, with
none of the heretofore mentioned drawbacks.
SUMMARY OF THE INVENTION
As used herein, the terms "composition(s) of the present
invention", or "composition(s) useful for making lyocell fibers",
or "treated pulp" refer to pulp, containing cellulose and
hemicellulose, that has been treated under alkaline conditions that
reduce the average degree of polymerization (D.P.) of the cellulose
without substantially reducing the hemicellulose content of the
pulp or substantially increasing the copper number for the pulp.
The compositions of the present invention preferably possess
additional properties as described herein.
Compositions of the present invention are compositions useful for
making lyocell fibers, or other molded bodies such as films, having
a high hemicellulose content, a medium to low copper number and
short fiber lengths, including cellulose that has a low average
D.P. Preferably, the cellulose and hemicellulose are derived from
wood, but more preferably from softwood. The present invention
preferably uses sawdust as a raw material, since being a byproduct
makes it economically more attractive than wood chips. Producing
wood chips on site at a mill is a capital intensive proposition.
However, the present invention may also use wood chips as a raw
material source. Additionally, the compositions of the present
invention exhibit a variety of desirable properties including a low
lignin content, a broad molecular weight distribution and a low
transition metal content. Compositions of the present invention may
be in a form that is adapted for storage or transportation, such as
in a sheet, roll or bale. Compositions of the present invention may
be mixed with other components or additives to form a dope useful
for making lyocell molded bodies, such as fiber or films. Further,
the present invention provides processes for making compositions
useful for making lyocell fibers having a desirable high
hemicellulose content, a medium to low copper number and short
fiber lengths, including cellulose that has a low average D.P.
The present invention also provides lyocell fibers containing
cellulose having a low average D.P., a high proportion of
hemicellulose short fibers, and a medium to low copper number, a
broad molecular weight distribution, and a low lignin content. The
lyocell fibers of the present invention also preferably possess a
low transition metal content.
Compositions of the present invention can be made in a continuous
digester, using an alkaline cook liquor, followed by an alkaline
D.P. reduction step. The cook liquor can be Kraft or soda.
Preferably the brownstock pulp is a Kraft softwood pulp, and still
more preferably a pulp made in an M&D or Pandia digester using
Kraft liquor and operating under conditions of short cook times and
high temperatures. Compositions of the present invention include at
least 7% by weight hemicellulose, preferably from 7% by weight to
about 35% by weight hemicellulose, more preferably from 7% by
weight to about 20% by weight hemicellulose, most preferably from
about 10% by weight to about 17% by weight hemicellulose, and
cellulose having an average D.P. of from about 200 to about 1100,
preferably from about 300 to about 1100, and more preferably from
about 400 to about 700. Compositions of the present invention can
have the combination of bemicellulose ranges and cellulose D.P.
ranges mentioned above in addition to having a copper number less
than about 2.0, wherein greater than 4% of the pulp fibers have a
length-weighted average fiber length less than 2.0 mm.
A first embodiment of a composition made according to the present
invention includes at least 7% hemicellulose, contains cellulose
having an average degree of polymerization of from about 200 to
about 1100, a copper member less than about 2.0, and greater than
4% of the pulp fibers have a length-weighted fiber length less than
2.0 mm. Hemicellulose content is measured by a sugar content assay
based on TAPPI Standard T249 hm-85. Further, compositions of the
present invention have a kappa number of less than 2, preferably
less than 1. Most preferably, compositions of the present invention
contain little to no detectable lignin. Lignin content is measured
using TAPPI Test T236 cm-85.
Compositions of the present invention preferably have a unimodal
distribution of cellulose D.P. values wherein the individual D.P.
values are approximately normally distributed around a single,
modal D.P. value, i.e., the modal D.P. value being the D.P. value
that occurs most frequently within the distribution. The
distribution of cellulose D.P. values may, however, be multimodal
i.e., a distribution of cellulose D.P. values that has several
relative maxima. A multimodal, treated pulp of the present
invention might be formed, for example, by mixing two or more
unimodal, treated pulps of the present invention that each have a
different modal D.P. value. The distribution of cellulose D.P.
values is determined by means of proprietary assays performed by
Thuringisches Institution fur Textil-und Kunstoff Forschunge. V.,
Breitscheidstr. 97, D-07407 Rudolstadt, Germany.
Compositions of the present invention have greater than 4% of pulp
fibers with a length-weighted fiber length of less than 2.0 mm,
owing to the raw material source, which can be sawdust or hardwood.
Length-weighted fiber length is the sum of fiber lengths divided by
the total length. The length-weighted fiber length is suitably
measured by a FQA machine, model No. LDA93-R704, with software
version 2.0, made by the Optest Company of Hawkesbury, Ontario,
Canada.
Compositions of the present invention which have been treated to
reduce their D.P. without substantially reducing the hemicellulose
content of the pulp, exhibit a desirably broad molecular weight
distribution as evidenced by a difference between R.sub.10 and
R.sub.18 values (.DELTA.R) of greater than or equal to about
2.8.
Additionally, compositions of the present invention preferably have
a relatively low carbonyl content as evidenced by a copper number
of less than about 2.0, more preferably less than about 1.6, as
measured by TAPPI Standard T430. Further, compositions of the
present invention preferably have a carbonyl content of less than
about 60 .mu.mol/g and a carboxyl content of less than about 60
.mu.mol/g, more preferably, a carbonyl content less than 30
.mu.mol/g and a carboxyl content less than about 30 .mu.mol/g. The
carboxyl and carbonyl group content are measured by means of
proprietary assays performed by Thuringisches Institut fur
Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407
Rudolstadt, Germany, referred to below as TITK.
Compositions of the present invention also preferably possess a low
transition metal content. Preferably, the total transition metal
content of the compositions of the present invention is less than
20 ppm, more preferably less than 10 ppm, as measured by
Weyerhaeuser Test Number AM5-PULP-1/6010. The term "total
transition metal content" refers to the combined amounts, measured
in units of parts per million (ppm), of nickel, chromium,
manganese, iron and copper. Preferably the iron content of the
compositions of the present invention is less than 8 ppm, more
preferably less than 4 ppm, as measured by Weyerhaeuser Test
AM5-PULP-1/6010, and the copper content of the compositions of the
present invention is preferably less than 1.0 ppm, more preferably
less than 0.5 ppm, as measured by Weyerhaeuser Test
AM5-PULP-1/6010.
Compositions of the present invention are readily soluble in amine
oxides, including tertiary amine oxides such as NMMO or NMMO
monohydrate. Other preferred solvents that can be mixed with NMMO,
or another tertiary amine solvent, include dimethylsulfoxide
(D.M.S.O.), dimethylacetamide (D.M.A.C.), dimethylformamide
(D.M.F.) and caprolactan derivatives. Preferably, compositions of
the present invention fully dissolve in NMMO monohydrate in less
than about 10 minutes, preferably in about 5 minutes or less or
even within 2 minutes, utilizing the dissolution procedure
described in Example 2 below. The term "fully dissolve", when used
in this context, means that substantially no undissolved particles
are seen when a dope, formed by dissolving compositions of the
present invention in NMMO, is viewed under a light microscope at a
magnification of 40.times. to 100.times..
A first preferred embodiment of the treated pulp of the present
invention is a treated alkaline pulp including at least 7% by
weight hemicellulose, a copper number less than about 2.0,
cellulose having an average degree of polymerization of from about
200 to about 1100, and greater than 4% of the pulp fibers have an
average length-weighted fiber length of less than 2.0 mm.
Lyocell fibers formed from compositions of the present invention
include at least about 5% by weight hemicellulose, preferably from
about 5% by weight to about 22% by weight hemicellulose, more
preferably from about 5% by weight to about 18% by weight
hemicellulose, most preferably from about 10% by weight to about
15% by weight hemicellulose, cellulose having an average D.P. of
from about 200 to about 1100, more preferably from about 300 to
about 1100, most preferably from about 400 to about 700, and a
lignin content providing a kappa number less than about 2.0 and
more preferably less than about 1.0. Lyocell fibers of the present
invention are made from a treated pulp having greater than 4% of
pulp fibers having a length-weighted fiber length less than 2.0 mm.
Additionally, lyocell fibers of the present invention can have a
unimodal distribution of cellulose D.P. values, although lyocell
fibers of the present invention may also have a multimodal
distribution of cellulose D.P. values, i.e., a distribution of
cellulose D.P. values that has several relative maxima. Lyocell
fibers of the present invention having a multimodal distribution of
cellulose D.P. values might be formed, for example, from a mixture
of two or more unimodal, treated pulps of the present invention
that each have a different modal D.P. value. Lyocell fibers of the
present invention can have a .DELTA.R of greater than or about
equal to 2.8.
Preferred lyocell fibers of the present invention have a copper
number of less than about 2.0, more preferably less than about 1.6,
as measured by TAPPI Standard T430. Further, preferred lyocell
fibers of the present invention have a carbonyl content of less
than about 60 .mu.mol/g and a carboxyl content of less than about
60 .mu.mol/g, more preferably a carbonyl content less than about 30
.mu.mol/g and a carboxyl content of less than about 30 .mu.mol/g.
The carboxyl and carbonyl group content are measured by means of
proprietary assays performed by Thuringisches Institut fur
Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407
Rudolstadt, Germany. Additionally, lyocell fibers of the present
invention have a total transition metal content of less than about
20 ppm, more preferably less than about 10 ppm, as measured by
Weyerhaeuser Test Number AM5-PULP-1/6010. The term "total
transition metal content" refers to the combined amount, expressed
in units of parts per million (ppm), of nickel, chromium,
manganese, iron and copper. The iron content of lyocell fibers of
the present invention is less than about 8 ppm, more preferably
less than about 4 ppm, as measured by Weyerhaeuser Test
AM5-PULP-1/6010, and the copper content of lyocell fibers of the
present invention is preferably less than about 1 ppm, more
preferably less than about 0.5 ppm, as measured by Weyerhaeuser
Test AM5-PULP-1/6010.
Preferred embodiments of the lyocell fibers of the present
invention possess desirable elongation properties. Lyocell fibers
of the present invention possess a dry elongation of from about 8%
to about 20%. Lyocell fibers of the present invention possess a wet
elongation of from about 10% to about 18%. Elongation is measured
by means of proprietary assays performed by Thuringisches Institut
fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407
Rudolstadt, Germany. Lyocell fibers produced from treated pulps of
the present invention have exhibited dry tenacities on the order of
about 20-48 cN/tex and wet tenacities on the order of 18-40 cN/tex
as measured by the proprietary assays performed by Thuringisches
Institut fur Textil-und Kunstoff Forschunge.
In another aspect, the present invention provides processes for
making compositions of the present invention that can, in turn, be
formed into lyocell molded bodies, such as fibers or films. In this
aspect, the present invention provides a process that includes
pulping a raw material in a digester to provide a brownstock
alkaline pulp, wherein the raw material comprises sawdust in an
amount greater than 0% up to 100%, and then contacting the alkaline
pulp comprising cellulose and at least about 7% hemicellulose under
alkaline conditions with an amount of an oxidant sufficient to
reduce the average D.P. of the cellulose to within the range of
from about 200 to about 1100, preferably to within the range of
from about 300 to about 1100, more preferably to within the range
of from about 400 to about 700, without substantially reducing the
hemicellulose content or increasing the copper number. Pulps which
are to be treated according to the present invention with an
oxidant to achieve the D.P. reduction without substantially
reducing the hemicellulose content or increasing the copper number
as discussed above preferably have a kappa number less than 40,
more preferably less than 30 and most preferably less than 25 when
they are contacted for the first time with the oxidant. The
hemicellulose content is reduced by less than 50%, more preferably
by less than 15% and most preferably by less than 5%. The copper
number increases by less than 50%, and more preferably by less than
about 25%. The reduction in the average degree of polymerization of
the cellulose can occur in the presence of a ratio of magnesium to
transition metals of less than 50%.
This D.P. reduction treatment can occur after the pulping process
and before, during or after a bleaching step, if a bleaching step
is utilized. The oxidant under alkaline conditions is any oxidant
containing a peroxide group such as hydrogen peroxide, oxygen,
chlorine dioxide and ozone and any combination thereof. Preferably
the oxidant is a combination of oxygen and hydrogen peroxide, or
hydrogen peroxide alone. A further step can include contacting the
pulp with an alkali source, such as sodium hydroxide, oxidized
white liquor, and unoxidized white liquor and any combination
thereof, at or above a pH of about 8.0. After the step of
contacting the pulp and oxidant to make a composition of the
present invention, the composition can be dissolved in any of the
aforementioned solvents to form a dope. The compositions of the
present invention are readily dissolved in NMMO-monohydrate within
5 minutes, preferably within about 2 minutes. The process of the
present invention can be carried out in a continuous digester
capable of handling sawdust, such as an M&D or Pandia-type
digester; and therefore, sawdust in any amount greater than 0% to
up to 100% of the raw material can be used. Alternatively, wood
chips having short fiber lengths, such as hardwoods, may also be
used. A suitable raw material sawdust contains less than or about
50% of fibers retained on a one-quarter inch screen, as measured by
TAPPI standard 233, preferably less than 30%, and more preferably
less than 15%. Alternatively, a combination raw material of sawdust
and wood chips can be used where the sawdust makes up about 50% to
about 100% of the raw material. However, wood chips can make up the
whole of the raw material provided the fiber tracheids are
comparable to sawdust fiber tracheids.
Preferably the yield of the D.P. reducing step of the present
invention is greater than about 95%, more preferably greater than
about 98%. The process yield is the dry weight of the treated pulp
produced by the process divided by the dry weight of the starting
material pulp, the resulting fraction being multiplied by one
hundred and expressed as a percentage (this percentage does not
include the amount of lignin loss during the stage).
In another aspect of the present invention, a process for making
lyocell fibers includes the steps of (a) pulping a raw material in
a digester to provide an alkaline pulp wherein the raw material
comprises sawdust in an amount greater than 0% up to 100%; (b)
contacting the alkaline pulp including cellulose and at least about
7% hemicellulose under alkaline conditions with an amount of an
oxidant sufficient to reduce the average degree of polymerization
of the cellulose to the range of from about 200 to about 1100,
preferably to the range of from about 200 to about 1100, without
substantially reducing the hemicellulose content or increasing the
copper number of the pulp; and (c) forming fibers from the pulp
treated in accordance with step (b). In accordance with this aspect
of the present invention, the lyocell fibers are preferably formed
by a process selected from the group consisting of melt blowing,
centrifugal spinning, spun-bonding and a dry jet/wet process.
The process uses an oxidant selected from the group consisting of a
chemical with a peroxide group, hydrogen peroxide, oxygen, chlorine
dioxide, ozone, and any combination thereof. The step of reducing
the average degree of polymerization of the cellulose occurs in the
presence of a ratio of magnesium to transition metals of less than
about 50%. The hemicellulose content is reduced less than about
15%, the copper number increases less than 25%, and the contacting
step can occur in the substantial absence of an inhibitor to
degradation of cellulose by the oxidant. The step of contacting can
further occur at a pH of greater than about 8.0. A further step of
the process can include dissolving the pulp in a solvent before
forming the fiber. The characteristic of the pulp being that it is
readily dissolving in NMMO-monohydrate within about 10 minutes by
the test performed in Example 3 of the present specification.
In a preferred embodiment of a process according to the present
invention, sawdust is used as the raw material, and the sawdust is
digested in an M&D or Pandia continuous digester using short
cook times and high temperatures to produce an alkaline pulp.
Sawdust is a readily available, often-discarded byproduct.
Combining a continuous sawdust pulping process with a pulp post
treatment according to the invention, activated dissolving type
pulps can be made more economically than by conventional high alpha
dissolving pulps. Thus fulfilling a need to conserve dwindling
landfill space while at the same time preserving natural resources
through the use of sawdust. Pulps and lyocell fibers according to
the invention can be made by any raw material having as much as
100% sawdust. The sawdust to make the pulp and fibers is measured
by TAPPI Standard 233, and can include about 50% of fibers retained
on a 1/4 inch mesh screen.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this
invention will become more readily appreciated as the same become
better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
FIG. 1 is a schematic diagram of the presently preferred process
for converting a raw material, preferably sawdust, to a composition
of the present invention useful for making lyocell molded
bodies;
FIG. 2 is a schematic diagram of the steps of the presently
preferred process of forming fibers from the compositions of the
present invention;
FIG. 3 is a micrograph of a wood chip pulp fiber made by the
process of application Ser. No. 09/574,538;
FIG. 4 is a micrograph of the fiber of FIG. 3 after 2 minutes of a
dissolution test;
FIG. 5 is a micrograph of a sawdust pulp fiber made by the process
according to the present invention; and
FIG. 6 is a micrograph of the fiber of FIG. 5 after 2 minutes of
the same dissolution test used in FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Heretofore, the size of the wood particles useful for making
dissolving pulps via an alkaline process was thought to be an
important parameter. The existing facilities of many pulping mills
making dissolving pulps have been designed with a particular sized
wood chip being the basis for the design. For example, most if not
all digesters presently making dissolving pulps out of wood chips
have a recycle stream to recycle black liquor to provide more
homogeneous conditions in the digester. In order to accomplish
this, a screen is necessary to prevent the wood chips from entering
the liquor recycle loop. The present digesters in these mills are
thus not suitable to accept small particulate wood matter, such as
sawdust, since the sawdust would clog the recycle screens and upset
the digester conditions. Thus, the process according to the present
invention makes use of a digester typically not considered useful
for making dissolving pulps and combines this digester with a pulp
treatment to make a composition useful for making lyocell molded
bodies. The digester is a continuous digester, such as a Pandia or
an M&D digester or other similarly designed type of digester.
These digesters typically do not require a recycle stream and thus
there are no screens to plug, making them suitable to pulp sawdust.
Continuous digesters typically have a feed nozzle at an upper
portion of the vessel and an exit nozzle at a lower portion.
However, other continuous digesters can have both the feed and exit
nozzle at the upper portion, lower portion or other suitable
arrangements. Continuous digesters are also typically, but not
always, operated at pressures above atmospheric pressure.
Pulping breaks down wood into its components of cellulose,
hemicellulose and lignin into more or less discrete fibers by
either chemical or mechanical methods. Once raw cellulose fibers
have been separated, further treatment in a bleach plant can define
one or more characteristics of the pulp. That is not to say,
however, that conditions in the pulping stage cannot affect the
properties of finished pulp. The source and size of the woody
material used to produce pulp also have an impact on the resulting
pulp product. Softwood includes wood from trees from the
subdivision of Spermatophytes, known as Gymnosperms, more commonly
known as conifers. Hardwood includes wood from trees from the
subdivision of Spermatophytes, known as Angiosperms, also called
broadleaf. Softwoods are characterized by fiber tracheids of
between about 2.5 to 7 mm long. Most softwoods have fiber tracheids
that average 3 to 3.6 mm, while hardwoods have fiber tracheids of
between about 0.9 to 1.5 mm. Pulps made from softwood are highly
desirable in the manufacture of lyocell articles. Fiber length adds
strength to the molded lyocell article. Sawdust, on the other hand,
is composed of small fiber tracheids, regardless of whether its
source is softwood or hardwood. Conventionally, it was thought
undesirable to use sawdust pulp for lyocell body formation, because
of the corresponding loss in strength. According to the present
invention, sawdust pulps can be manufactured which do have a short
fiber length, but nonetheless have been shown to produce suitable
lyocell molded bodies having suitable strength properties.
Sawdust is generally understood to be the small-particle wood
residue generated by lumber-cutting operations, such as by the use
of a saw blade. Other small wood particles such as chip screen
fines and pin chips are often referred to as sawdust as well. Small
particles generated either from lumber sawing operations or chip
screening operations are considered to be sawdust. Sawdust
generally has many impurities and residual metals because of
contact with saw or abrading equipment. Mostly, sawdust is used as
an absorbent, and has had little use as raw material for pulps.
When sawdust pulp is produced, the majority is pulped using either
of the aforementioned continuous processes. Unlike conventionally
sized wood chips, sawdust must be pulped in digesters specially
designed to handle smaller particle sizes. These digesters are
designed to have significant lower residence time, higher
liquor-to-wood ratios and higher cooking temperatures unlike
conventional digesters used to make dissolving pulps. This is
because the surface area of sawdust in comparison to wood chips can
be 10 to 50 times or greater, and the permeability of the sawdust
to the cooking liquid is very rapid. Suitable digesters to use in
the present invention are an M&D (Messing and Durkee) or Pandia
digester. Other digesters of similar design exist and may be also
used. Conventional digesters presently making dissolving pulp are
unsuited to handle the short cook times needed for sawdust.
Prior to undertaking the discussion of the methods and products of
the present invention, it is beneficial to describe the preferred
sizes of sawdust useful in the practice of the invention. As used
herein, "sawdust" includes any collection of small wood particles
from any known softwood or hardwood. The particle size distribution
of sawdust from lumber sawing operations is determined by sawing
variables such as saw thickness, saw speed, saw tooth design, and
saw diameter. As such, the size distribution will vary depending on
the design of the lumber sawing operation producing the
sawdust.
In addition to particles produced from sawn kerf in lumber
producing operations, sawdust also includes other small wood
particles such as chip screen fines and pin chips. Screen fines are
removed from wood chips prior to pulping by a screen having
openings of 3 15 mm diameter holes, interface or inter-roll
distances for blade, disk and roll screens, diagonal or rectangular
screen openings are also within about the same range.
Several authorities have put forth a quantitation method for
sawdust. In one method, the sawdust is classified by the fractions
of a sawdust sample successively retained on a particular mesh
size. An example of a sawdust sample measurement is given in the
following fractions and mesh screen sizes, where a positive (+)
sign means the percentage of the initial sample retained on the
screen, and a negative (-) sign means the percentage of the initial
sample that passed through the screen: +1/4 inch mesh is 2 to 4%,
+6 mesh is 4 to 5%, +10 mesh is 27 to 24%, +20 mesh 36 to 38%, and
-20 mesh is 28 to 32%. This means that 2 to 4% of the sawdust
sample did not pass through the 1/4 inch mesh screen, and so on.
The negative (-) sign indicates the percentage of smaller particles
passing through the 20 mesh screen. In another sawdust sample
measurement, the same authority quantifies the sawdust by TAPPI
standard T233, also commonly referred to as Bauer McNett, which
measures the fiber sizes from the sawdust. The sawdust sample
resulted in the following quantification: +1/4 inch screen is 4 to
18%, +35 screen is 12%, +65 screen is 49 57%, +150 screen is 17 to
22%, and -150 screen is 4 5. 1997 TAPPI Forest Biology Wood
Chemistry Conference, Walter J. Bublitz and Jerry L. Hull. Another
authority reported the following Bauer McNett fiber fractions of a
sawdust sample: 24.3% retained on 14 mesh, 26.7% retained on 28
mesh, 43.2% retained on 100 mesh, 5.2% retained on 200 mesh, and
0.6% passed through 200 mesh. For comparison only, pulp from mixed
hardwood chips contained the following fractions: 0.2% retained on
14 mesh, 12.8% retained on 28 mesh, 66.2% retained on 100 mesh,
6.6% retained on 200 mesh, and 14.2% passed through 200 mesh. Pulp
and Paper, Feb. 1979, Cecil M. Bail, pp. 105 109. Another authority
reported the following Tyler Sieve analysis for sawdust particles:
19.2% retained on 4 mesh, 28.0% retained on 8 mesh, 14.7% retained
on 10 mesh, 21.6% retained on 20 mesh, 10.5% retained on 35 mesh,
and 6.0% was collected in the pan. The same authority also uses
Bauer McNett to report fiber classification: +12 mesh is 1%, +28
mesh is 45%, +48 mesh is 24%, +100 mesh is 12%, and -100 is 18%.
For comparison only, the chip pulp fiber classification is: +12 is
47%, +28 is 34%, +48 is 8%, +100 is 3%, and -100 is 8%. Pulp and
Paper Canada, Jan. 1997, T. G. Taylor, pp. 53 57.
As used herein, sawdust will be defined to mean any collection of
wood particles from any softwood or hardwood having no more than a
50% fraction of a representative sample retained on a 1/4 inch
screen as measured by TAPPI standard T233. Sawdust may include
conventionally sized chips of all sizes provided that the fiber
sizes of the wood chips meet the fiber length classification for a
sawdust as defined. While the examples given above have been
described with reference to particular embodiments, they are not
meant to be limiting of the invention which makes use of a raw
material having small wood particles, which also may include any
quantity of conventionally sized wood chips as part of the 50%
fraction retained on a 1/4 mesh screen.
The raw material, such as sawdust, useful in the practice of the
present invention, contains cellulose and hemicellulose. Sources of
sawdust useful in the practice of the present invention include,
but are not limited to trees. The sawdust used in the practice of
the present invention, from whatever source, is initially converted
to a pulp using an alkaline pulping process, such as the Kraft or
soda process modified for an M&D, Pandia or other similar type
of continuous digester.
There are basically two types of reactors to be used for pulping,
batch and continuous. Conventional batch reactors containing a
recycle stream provide adequate pulping of wood chips, however, in
order to provide a recycle stream to provide a homogeneous mixture,
screens must be placed on the recycle line to prevent wood chips
from entering the pump. In order to produce a dissolving pulp from
sawdust, a way of avoiding the plugging of screens in the recycle
line had to be resolved. The method according to the present
invention solves this problem by providing a dissolving pulp from
sawdust using a continuous rector, such as M&D or Pandia. These
types of reactors, continuously remove liquor, and then either
recover or discard the used liquor unlike the current batch
processing of wood chips.
The preferred method for pulping the sawdust into the alkaline pulp
includes the use of a continuous digester such as an M&D or
Pandia digester, such as the one described in U.S. Pat. No.
3,586,600, which is herein incorporated by reference. These
digesters have been typically used in sawdust pulping, however, not
for making dissolving pulps. It is not beyond the scope of the
present invention to utilize a batch reactor as well, provided it
is suitable to accept the small wood particles found in sawdust.
The pulping step produces an alkaline chemical wood pulp,
preferably an unbleached continuously digested Kraft wood pulp,
containing cellulose and at least about 7% hemicellulose, that has
not been exposed to acid hydrolysis conditions or any other
heterogeneous mixture conditions (i.e., reaction time, temperature,
and acid concentration), where cellulose glycosidic bonds are
broken. Alternatively, a bleaching step can be employed before,
during and after the D.P. reduction step to produce a bleached
continuously digested Kraft wood pulp, containing cellulose and at
least 7% hemicellulose, that has not been exposed to acid
hydrolysis conditions or any other heterogeneous mixture
conditions, where cellulose glycosidic bonds are broken. Both
unbleached and bleached pulps can be further treated according to
the present invention to arrive at a composition useful for making
lyocell molded bodies. "Continuously digested" is being used to
signify that the pulp is being made in a continuous digester, such
as an M&D or Pandia or other similar digester, heretofore,
known to be used for pulping sawdust, but not as a method of making
dissolving pulp, which is an object of the present invention. The
present invention uses Kraft cooking liquor in the continuous
digester. The process according to the invention employs shorter
cook times and higher temperatures. Cook time will vary depending
on wood particle size, digester size, throughput rate, level and
strength of the cooking liquor and temperature. However, a suitable
range is on the order of about 30 to about 90 minutes; preferably,
on the order of 60 minutes. The cook temperatures will also vary
depending on the above-mentioned variables but a suitable
temperature is generally on the order of about 180.degree. C. or
higher. It should be readily apparent to one skilled in the art
that time and temperature can be lower or higher depending on the
variables set forth previously. The cooking liquor is similar to
that for a typical Kraft cook and includes a mixture of sodium
sulfide and sodium hydroxide. After the sawdust has been converted
to a continuously digested Kraft wood pulp in accordance with the
present invention, it is processed to reduce its average D.P.
without substantially reducing its hemicellulose content or
increasing its copper number according to the present
invention.
In the wood pulping industry, trees are conventionally classified
as either hardwood or softwood. In the practice of the present
invention, sawdust can be derived from softwood tree species such
as, but not limited to: fir (preferably Douglas fir and Balsam
fir), pine (preferably Eastern white pine and Loblolly pine),
spruce (preferably White spruce), larch (preferably Eastern larch),
cedar, and hemlock (preferably Eastern and Western hemlock).
Examples of hardwood species from which sawdust can also be derived
include, but are not limited to: acacia, alder (preferably Red
alder and European black alder) aspen (preferably Quaking aspen),
beech, birch, oak (preferably White oak), gum trees (preferably
eucalyptus and Sweetgum), poplar (preferably Balsam poplar, Eastern
cottonwood, Black cottonwood and Yellow poplar), gmelina and maple
(preferably Sugar maple, Red maple, Silver maple and Bigleaf
maple). Sawdust may also be derived from other sources containing
cellulose, which generates small particulate matter.
Wood from softwood or hardwood species generally includes three
major components: cellulose, hemicellulose and lignin. Cellulose
makes up about 50% of the woody structure of plants and is an
unbranched polymer of D-glucose monomers. Individual cellulose
polymer chains associate to form thicker microfibrils which, in
turn, associate to form fibrils which are arranged into bundles.
The bundles form fibers which are visible as components of the
plant cell wall when viewed at high magnification under a light
microscope. Cellulose is highly crystalline as a result of
extensive intermolecular and intramolecular hydrogen bonding.
The term hemicellulose refers to a heterogeneous group of low
molecular weight carbohydrate polymers that are associated with
cellulose in wood. Hemicelluloses are amorphous, branched polymers,
in contrast to cellulose which is a linear polymer. The principal,
simple sugars that combine to form hemicelluloses are: D-glucose,
D-xylose, D-mannose, L-arabinose, D-galactose, D-glucuronic acid
and D-galacturonic acid.
Lignin is a complex aromatic polymer and comprises about 30% to 50%
of wood where it occurs as an amorphous polymer.
In the pulping industry, differences in the chemistry of the
principal components of wood are exploited in order to purify
cellulose. For example, heated water in the form of steam causes
the removal of acetyl groups from hemicellulose with a
corresponding decrease in pH due to the formation of acetic acid.
At elevated temperatures of about 150.degree. C. 180.degree. C.,
acid hydrolysis of the carbohydrate components of wood then ensues,
with a lesser hydrolysis of lignin. Hemicelluloses are especially
susceptible to this acid hydrolysis, and most of the hemicellulose
can be degraded by an initial steam, prehydrolysis step in the
Kraft pulping process, as described in the Background, or in an
acidic sulfite cooking process. However, the removal of significant
quantities of hemicellulose is disfavored in the present
invention.
With respect to the reaction of wood with alkali solutions, all
components of wood are susceptible to degradation by strong
alkaline conditions. At the elevated temperature of 140.degree. C.
or greater that is typically utilized during Kraft wood pulping,
the hemicelluloses and lignin are preferentially degraded by dilute
alkaline solutions. In continuous digesters, such as an M&D or
Pandia digester, the temperature is much higher, being on the order
of about 180.degree. C. to about 190.degree. C. The high
temperature leads to rapid degradation and a corresponding decrease
in cellulose D.P. This is advantageous from the standpoint of
achieving a dissolving pulp. Additionally, all components of wood
can be oxidized by bleaching agents such as chlorine, sodium
hypochlorite and hydrogen peroxide.
The alkaline pulping step that is carried out in a continuous
digester can be used to provide a continuously digested brownstock
alkaline pulp that can be further treated as explained below to
provide an alkaline pulp useful for making lyocell fibers.
A typical Kraft bleaching sequence can contain a chlorine dioxide
stage or multiple chlorine dioxide stages with a pH less than 4 and
a temperature greater than about 70.degree. C., but the combined
heterogeneous mixture conditions (i.e., reaction time, temperature
and acid concentration) of such stages are not suitable to induce
substantial DP reduction in cellulose. By avoiding an acid
pretreatment step prior to alkaline pulping, according to the
invention, the overall cost of producing the alkaline pulps is
reduced. Further, by avoiding acid prehydrolysis, the degradation
of hemicellulose is averted and the overall yield of the pulping
process can be increased. By employing a continuous digester,
inexpensive sources of cellulose and hemicellulose, such as
sawdust, may be used to produce a pulp suitable for conversion into
lyocell molded bodies. As used herein the phrase alkaline pulp
refers to pulp containing cellulose and hemicellulose that has not
been subjected to any combination of acidic conditions or any other
heterogeneous mixture conditions that would result in breaking of
the cellulose glycosidic bonds before or during the pulping process
wherein the sawdust is converted to fibers.
Characteristics of a continuously digested brownstock alkaline pulp
suitable for use in the D.P. reduction step of the present
invention include a pulp having a hemicellulose content of at least
7% by weight, preferably from 7% to about 30% by weight, more
preferably from 7% to about 25% by weight, and most preferably from
about 9% to about 20% by weight; an average D.P. of cellulose of
from about 600 to about 1800; a kappa number less than about 40
preferably less than 30 and more preferably less than 25, and a
copper number less than about 2.0. As used herein, the term
"percent (or %) by weight" or "weight percent", or grammatical
variants thereof, when applied to the hemicellulose or lignin
content of pulp, means weight percentage relative to the dry weight
of the pulp.
Referring to FIG. 1, the process for providing a composition
according to the present invention is illustrated. The process
includes a block 100 for pulping a raw material, the manner of
pulping previously indicated as using a continuous digester.
Hereinafter, raw material(s) will mean the cellulosic materials
introduced into the front end of the process prior to pulping. The
raw materials may undergo further treatment or processing before
they are fed into the pulping digester 100. Provided, that the raw
materials do not undergo any form of treatment that substantially
reduces the hemicellulose content. From the digester, the pulp can
optionally be washed or screened. The Brownstock pulp is further
treated according to the invention to reduce its D.P. without
substantially reducing hemicellulose, in a reactor whereby the
average D.P. of the cellulose is reduced, without substantially
reducing the hemicellulose content or increasing the copper number,
to provide the alkaline pulps of the present invention in block
104. In this context, the term "without substantially reducing the
hemicellulose content" means without reducing the hemicellulose
content by more than about 50%, preferably not more than about 15%,
and most preferably not more than about 5% during the D.P.
reduction step. The term "degree of polymerization" (abbreviated as
D.P.) refers to the number of D-glucose monomers in a cellulose
molecule. Thus, the term "average degree of polymerization", or
"average D.P.", refers to the average number of D-glucose molecules
per cellulose polymer in a population of cellulose polymers. This
D.P. reduction treatment can occur after the pulping process in
block 102 and before, after or substantially simultaneously with
the bleaching process, if a bleaching step is utilized, in blocks
102, 108, and 106 respectively. In this context, the term
"substantially simultaneously with" means that at least a portion
of the D.P. reduction step occurs at the same time as at least a
portion of the bleaching step. Preferably the average D.P. of the
cellulose is reduced to a value within the range of from about 200
to about 1100; more preferably to a value within the range of from
about 300 to about 1100; most preferably to a value of from about
400 to about 700. Unless stated otherwise, D.P. is determined by
ASTM Test 1301 12. A D.P. within the foregoing ranges is desirable
because, in the range of economically attractive operating
conditions, the viscosity of the dope, i.e., the solution of
treated pulp from which lyocell fibers are produced, is
sufficiently low that the dope can be readily extruded through the
narrow orifices utilized to form lyocell fibers, yet not so low
that the strength of the resulting lyocell fibers is substantially
compromised. Preferably the range of D.P. values of the treated
pulp will be unimodal and will have an approximately normal
distribution that is centered around the modal D.P. value.
Copper number is a measure of the carboxyl content of pulp. The
copper number is an empirical test used to measure the reducing
value of cellulose. The copper number is expressed in terms of the
number of milligrams of metallic copper which is reduced from
cupric hydroxide to cuprous oxide in an alkaline medium by a
specified weight of cellulosic material.
In this application, the term "without substantially increasing the
copper number" means without increasing the copper number by more
than about 100%, preferably not more than about 50% and most
preferably not more than about 25% during the D.P. reduction step.
The degree to which the copper number changes during the D.P.
reduction step is determined by comparing the copper number of the
digested pulp entering the D.P. reduction step and the copper
number of the treated pulp after the D.P. reduction step. A low
copper number is desirable because it is generally believed that a
high copper number causes cellulose and solvent degradation during
and after dissolution of the treated pulp to form a dope. However,
there is some evidence to suggest that the carbonyl content of
pulps, such as those of the present invention, containing a
relatively high amount of branched polymers, such as
hemicelluloses, do not see appreciable solvent degradation, even
with a high copper number. It is believed that the carbonyls at the
chain ends do not appreciably cause solvent degradation, even with
a high copper number. Since a high hemicellulose pulp has many
chain ends with corresponding carbonyls, a much higher carbonyl
content and thus copper number can be tolerated for these high
hemicellulose pulps as opposed to high alpha pulps. Thus, the
carbonyl content and copper number can be higher for high
hemicellulose pulps as opposed to high alpha pulps. It is also
generally assumed that a high copper number results in a less
thermally stable pulp; however, this assumption may eventually be
proved incorrect. In some extremely low D.P. pulps, of about 190 to
450 D.P., a copper number greater than 2, may not be a true
indication of thermal instability. Pulps with copper numbers as
high as 4 can still be thermally stable, provided, the copper
number is more a measure of the carbonyl content of the chain ends,
rather than the carbonyl content between chain ends.
The hemicellulose content of the treated alkaline pulp of the
present invention, expressed as a weight percentage is at least 7%
by weight; preferably from about 7% by weight to about 25% by
weight; more preferably from about 7% by weight to about 20% by
weight; most preferably from about 10% by weight to about 17% by
weight. As used herein, the term "percent (or %) by weight" or
"weight percentage", or grammatical equivalents thereof, when
applied to the hemicellulose or lignin content of treated pulp,
means weight percentage relative to the dry weight of the treated
pulp.
The pulps of the present invention have greater than 4% of fibers
with a length-weighted fiber length of less than 2.0 mm. This is
attributed, in part, from the raw material used, such as sawdust or
hardwood.
The pulps of the present invention exhibit an .DELTA.R of greater
than or about equal to 2.8. Treated pulps of U.S. application Ser.
No.09/574,538, which is herein incorporated by reference, exhibit a
.DELTA.R of less than 2.8. Pulps in accordance with the teachings
of U.S. application Ser. No. 09/256,197 exhibit a .DELTA.R of
greater than 2.8, however, also have a copper number exceeding 2.0.
After treatment to reduce the copper number to below 2.0 in
accordance with the '197 application, the .DELTA.R for the pulps of
prior application '197 is reduced to less than about 2.8.
R.sub.10 refers to the residual undissolved material that is left
after attempting to dissolve the pulp in a 10% caustic solution.
R.sub.18 refers to the residual amount of undissolved material left
after attempting to dissolve the pulp in an 18% caustic solution.
Generally, in a 10% caustic solution, hemicellulose and chemically
degraded short chain cellulose are dissolved and removed in
solution. In contrast, generally only hemicellulose is dissolved
and removed in an 18% caustic solution. Thus, the difference
between the R.sub.10 value and the R.sub.18 value represents the
amount of chemically degraded short chained cellulose that is
present in the pulp sample. Providing a pulp having a relatively
broad molecular weight distribution of at least equal to or greater
than about 2.8 is desirable from the standpoint of being able to
provide customers with pulp which may not require blending with
pulps of other molecular weight distribution to arrive at the
desired composition.
Without intending to be bound by theory, it is believed that the
chemical form of the hemicellulose in pulps treated in accordance
with the present invention is distinct from the chemical form of
hemicellulose in pulps that have been exposed to acidic conditions
or heterogeneous mixture conditions described above which result in
the breaking of cellulose glycosidic bonds, such as the pulps
described in prior application Ser. No. 09/256,197 and commercially
available dissolving grade pulps. This difference in chemical form
may be evidenced by the D.P. of the hemicellulose in the pulp of
the present invention compared to the D.P. of the hemicellulose of
the pulp of the prior application or commercial dissolving grade
pulps. This D.P. difference can be observed when the respective
pulps are derivatized (acetylated) and tested in the accordance
with the discussion by S. A. Rydholm in Pulping Processes,
Interscience Publishers, 1965. The higher D.P. hemicellulose in
treated alkaline pulps of the present invention may be less likely
to be extracted from lyocell filaments during the filament
formation process or post treatment of the formed lyocell filament
as compared to the hemicellulose of the pulps of the prior
application '197 or commercially available dissolving grade
pulps.
A presently preferred method of treating pulp in order to reduce
the average D.P. of the cellulose without substantially reducing
the hemicellulose content of the pulp and without substantially
increasing the copper number of the pulp is to treat the pulp under
alkaline conditions in high consistency or medium consistency
reactor(s) where the pulp is contacted with an oxidant containing a
peroxide group such as oxygen, hydrogen peroxide, chlorine dioxide,
ozone or combinations thereof. Preferably the oxidant is a
combination of oxygen and hydrogen peroxide or hydrogen peroxide
alone. This treatment step has been described in the patent
application Ser. No. 09/574,538. The present invention is the
addition of a continuously digested brownstock pulp from sawdust to
provide a more economical route to a pulp useful in lyocell
production.
The pulps formed in accordance with the present invention which
have been treated in order to reduce their average degree of
polymerization values without substantially decreasing the
hemicellulose content or the copper number for the pulp can be
produced by contacting the pulp in a reactor with an oxidant under
conditions suitable to achieve the desired results described above.
Suitable reactors include reactors conventionally used as oxygen
reactors in a Kraft process. Examples of reactors capable of
carrying out the contacting of the pulp with the oxidant are
described in U.S. Pat. Nos. 4,295,925; 4,295,926; 4,298,426;
4,295,927, each of which is herein incorporated by reference.
Unlike conventional oxygen reactors which are configured and
operated under conditions that preferably do not decrease the
average degree of polymerization of cellulose while at the same
time remove lignin, applicants' invention is designed to operate a
reactor under conditions that reduce the average degree of
polymerization of the cellulose without substantially reducing the
hemicellulose content or increasing the copper number of the
cellulose. In accordance with the present invention, the reactor
can be a high consistency reactor wherein the consistency of the
feedstream to the reactor is greater than about 20% or it can be a
medium consistency reactor where the consistency ranges between
about 8% up to about 20%. The conditions under which a high
consistency reactor or a medium consistency reactor is typically
operated in order to achieve the desired results of the present
invention relate primarily to operation of the high consistency
reactor at a temperature that is slightly higher than the
temperature at which the medium consistency reactor can be operated
as described below in more detail.
Examples of oxidants that can be employed have been described
above. Preferred oxidants include hydrogen peroxide alone or a
combination of oxygen and hydrogen peroxide. The amount of oxidant
employed should provide the desired D.P. reduction and lignin
removal given the time and temperature conditions employed.
Examples of suitable ranges for oxygen and hydrogen peroxide are
given below. Preferably, for a high consistency reactor, the oxygen
is present in an amount ranging from about 0 to the maximum
pressure rating for the reactor, preferably about 0 to about 85
psig, and more preferably, from about 40 to about 60 psig. The
hydrogen peroxide may be present in an amount ranging from greater
than about 0.75 weight percent up to about 5.0 weight percent, more
preferably about 1.0 to about 2.5 weight percent.
In medium consistency reactors, the oxygen can be present in an
amount ranging from about 0 to about 100 pounds per ton of the
pulp, more preferably, about 50 to about 80 pounds per ton of pulp.
The hydrogen peroxide may be present in an amount ranging from
greater than about 0.75 weight percent up to about 5 weight
percent, more preferably from about 1.0 to about 2.5 weight
percent.
In addition to the oxidants, caustic is preferably contacted with
the pulp in the reactor as a buffering agent. The source of caustic
can be sodium hydroxide or other materials such as unoxidized white
liquor or oxidized white liquor. The amount of caustic added will
depend in part upon the kappa number of the untreated pulp.
Generally, as the kappa number increases, more caustic is added.
The amount of caustic introduced can vary depending on process
conditions, with an amount of 4 to 5 weight percent or greater
being suitable.
The temperature at which the reactor is operated will in part
depend upon the concentration of the oxidants. When the oxidants
are used in amounts that fall within the ranges described above,
temperatures on the order of about 80.degree. C. up to about
130.degree. C. are suitable. It should be understood that the
temperature in the reactor may vary over time as the reactions that
occur therein tend to be exothermic which will most likely result
in an increase of the temperature of the reactor. It should be
understood that temperatures and oxidant concentrations falling
outside the ranges described above may still provide suitable
results depending on the various permutations of the amounts of
oxidant used and the temperatures.
In accordance with the present invention, the stage or stages used
to reduce the average degree of polymerization of the pulp without
substantially decreasing the hemicellulose content or increasing
the copper number of the pulp remains alkaline through the stage or
stages. Preferably, the pH of the stage or stages used to achieve
the D.P. reduction described above is greater than about 8.0
throughout the D.P. reduction process. It should be understood that
pHs above or below the noted ranges may provide satisfactory
results if the temperature or concentration of oxidant is modified
as necessary.
A presently preferred method of treating pulp in order to reduce
the average D.P. of the cellulose without substantially reducing
the hemicellulose content of the pulp and without substantially
increasing the copper number of the pulp is to treat the pulp in a
series of stages. In one embodiment, the pulp undergoes treatment
in a DEDE sequence. E stages are the principal stages used in
lowering the average D.P. of the cellulose without substantially
reducing the hemicellulose content of the pulp or raising the
copper number.
In the first D stage, the pulp consistency is adjusted to about 10%
with the addition of water. Chlorine dioxide corresponding to an
amount equivalent to about 28.4 pounds per ton of pulp is added to
the dissolved pulp. The mixture is held at a temperature of about
75.degree. C. for about 1.5 hours.
In the second E stage, the pulp consistency is maintained at about
10% with the addition of water. Sodium hydroxide was charged to the
reactor in an amount equivalent to about 30 pounds per ton of pulp.
Hydrogen peroxide was charged to the reactor in an amount
equivalent to about 60 pounds per ton of pulp. The mixture is held
for about 1.5 hours at a temperature of about 88.degree. C.
In the third D stage, chlorine dioxide is charged to the pulp in an
amount equivalent to about 19 pounds per ton, and the pulp was
again diluted to bring the consistency to about 10%. The mixture
was held for 1.5 hours at about 75.degree. C.
In the fourth EP stage, sodium hydroxide was charged to the pulp
with water being added to achieve a consistency of about 10%. The
sodium hydroxide charge was equivalent to about 30 pounds per ton
of pulp. Hydrogen peroxide is also charged in an amount equivalent
of about 40 pounds per ton. The mixture is held for about 1.5 hours
at a temperature of about 88.degree. C.
In accordance with the present invention, it is preferred that
contact between the pulp and the oxidant occur prior to any acid
wash or chelation stage normally used to remove transition metals.
Unlike prior art processes which intentionally sought to remove
transition metals which were believed to result in decomposition of
hydrogen peroxide into cellulose-degrading intermediates that
negatively impacted the viscosity of the cellulose, applicants have
discovered that they can take advantage of the presence of
naturally occurring transition metals in the wood to partially
degrade the hydrogen peroxide to produce intermediates that react
with the cellulose to reduce its average degree of polymerization
without substantially decreasing the hemicellulose content or
increasing the kappa number. In addition, unlike prior art
processes that use magnesium sulfate as a means of inhibiting the
degradation of cellulose, applicants prefer not to introduce
magnesium sulfate into the reactor or upstream therefrom so that
the pulp is contacted with the oxidant(s) in the substantial
absence of an inhibitor to the degradation of the cellulose by the
oxidant. If magnesium sulfate is present in the pulp prior to the
reactor, it is preferred that the ratio of magnesium to the
transition metals be less than 50% on a weight percent basis.
When a brownstock sawdust wood pulp containing cellulose and at
least 7% hemicellulose having a copper number of about 2.0 or less
is contacted with an oxidant under the conditions set forth above,
a treated pulp is produced having a D.P. ranging from about 200 to
about 1,100, containing at least 7% by weight hemicellulose, having
a copper number less than about 2.0 and a percentage of fibers
which have a length weighted average fiber length of about 2.0 mm,
of greater than 4%. It should be understood that the description
above of particular conditions under which a bleached or unbleached
wood pulp can be contacted with an oxidant to reduce its average
degree of polymerization without substantially reducing the
hemicellulose content or increasing the copper number are exemplary
and that other conditions can provide suitable results and still
fall within the scope of the present invention. In addition, it
should be understood that in some situations, the pulp exiting the
D.P. reduction stage may be suitable for use in producing a dope
for manufacture of lyocell fibers; however, in other situations,
subsequent process stages such as bleaching stages may be desirable
provided that subsequent stages do not result in a significant
decrease in the hemicellulose content or a significant increase in
the copper number of the pulp. In addition, as noted above, in some
situations, it may be necessary or advantageous to subject the pulp
which has been exposed to an oxidant in a first stage to a second,
third or even more stages of contact with an oxidant in order to
further reduce the degree of polymerization of the cellulose
without substantially reducing the hemicellulose content or
increasing the copper number thereof.
Again with reference to FIG. 1, once the alkaline pulp has been
treated with oxidants in a reactor in accordance with the present
invention, the treated pulp can either be washed in water and
transferred to a bath of organic solvent, such as NMMO, for
dissolution prior to lyocell molded body formation, block 112.
Alternatively, the treated washed pulp can be dried and broken into
fragments for storage and/or shipping in block 110.
A desirable feature of the treated pulps of the present invention
is that the cellulose fibers remain substantially intact after
treatment. Consequently, the treated pulp has a freeness and a
fines content that are similar to those of the untreated pulp.
Another desirable feature of the treated pulps of the present
invention is their solubility in organic solvents, such as tertiary
amine oxides including NMMO or NMMO monohydrate. Rapid
solubilization of the treated pulp prior to spinning lyocell fibers
is important in order to reduce the time required to generate
lyocell fibers, or other molded bodies such as films, and hence
reduce the cost of the process. Further, efficient dissolution is
important because it minimizes the concentration of residual,
undissolved particles, and partially dissolved, gelatinous
material, which can reduce the speed at which fibers can be spun,
tend to clog the spinnerets through which lyocell fibers are spun,
and may cause breakage of the fibers as they are spun. Conventional
pulps, on the other hand, must be made more soluble by chemical
additives. On the contrary, the treated pulps produced according to
the present invention are dissolved readily within at least 60
minutes under some circumstances, depending on the type of
solvent.
While not wishing to be bound by theory, it is believed that the
processes of the present invention utilized to reduce the average
D.P. of the cellulose also permeabilize the secondary layer of the
pulp fibers, thereby permitting the efficient penetration of
solvent throughout the pulp fiber. The secondary layer is the
predominant layer of the cell wall and contains the most cellulose
and hemicellulose. The short fibers attributed to sawdust pulp also
enhance the penetration of solvent into the fiber. A strong synergy
between mechanical macro structure damage to the raw material and
chemical nature (dissolution rate or activation) of the pulp
produced from this raw material exists.
Further, compositions of the present invention preferably have a
carbonyl content of less than about 60 .mu.mol/g and a carboxyl
content of less than about 60 .mu.mol/g, more preferably, a
carbonyl content of less than about 30 .mu.mol/g and a carboxyl
content of less than 30 .mu.mol/g. The carboxyl and carbonyl group
content are measured by means of proprietary assays performed by
Thuringisches Institut fur Textil-und Kunstoff Forschunge. V.,
Breitscheidstr. 97, D-07407 Rudolstadt, Germany. As an alternative
to determining the carbonyl content of the pulp using the
proprietary TITK assays, pulp samples and a thermally stable,
low-carbonyl group pulp can be analyzed by FTIR and the differences
in the spectrums between the two samples can provide an indication
of the existence of carbonyl groups and the thermal stability of
the pulp.
Additionally, the treated pulp of the present invention preferably
has a low transition metal content. Transition metals are
undesirable in treated pulp because, for example, they accelerate
the degradation of cellulose and NMMO in the lyocell process.
Examples of transition metals commonly found in treated pulp
derived from trees include iron, copper, nickel and manganese.
Preferably, the total transition metal content of the compositions
of the present invention is less than about 20 ppm, more preferably
less than about 10 ppm. Preferably the iron content of the
compositions of the present invention is less than about 8 ppm,
more preferably less than about 4 ppm, as measured by Weyerhaeuser
Test AM5-PULP-1/6010, and the copper content of the compositions of
the present invention is preferably less than about 1.0 ppm, more
preferably less than about 0.5 ppm, as measured by Weyerhaeuser
Test AM5-PULP-1/6010.
In order to make lyocell fibers, or other molded bodies, such as
films, from the treated pulp of the present invention, the treated
pulp is first dissolved in an amine oxide, preferably a tertiary
amine oxide. Representative examples of amine oxide solvents useful
in the practice of the present invention are set forth in U.S. Pat.
No. 5,409,532. The presently preferred amine oxide solvent is
N-methyl-morpholine-N-oxide (NMMO). Other representative examples
of solvents useful in the practice of the present invention include
dimethylsulfoxide (D.M.S.O.), dimethylacetamide (D.M.A.C.),
dimethylformamide (D.M.F.) and caprolactan derivatives. The treated
pulp is dissolved in amine oxide solvent by any art-recognized
means such as are set forth in U.S. Pat. Nos. 5,534,113; 5,330,567
and 4,246,221. The dissolved, treated pulp is called dope. The dope
is used to manufacture lyocell fibers, or other molded bodies, such
as films, by a variety of techniques, including melt blowing,
spun-bonding, centrifugal spinning, dry-jet wet, and other methods.
Examples of techniques for making a film from the compositions of
the present invention are set forth in U.S. Pat. No. 5,401,447 to
Matsui et al., and in U.S. Pat. No. 5,277,857 to Nicholson.
One useful technique for making lyocell fibers from dope involves
extruding the dope through a die to form a plurality of filaments,
washing the filaments to remove the solvent, and drying the lyocell
filaments. FIG. 2 shows a block diagram of the presently preferred
process for forming lyocell fibers from the treated pulps of the
present invention. The term "cellulose" in block 200 refers to the
compositions of the present invention. If necessary, the cellulose
in the form of treated pulp is physically broken down, for example
by a shredder in block 202, before being dissolved in an amine
oxide-water mixture to form a dope, blocks 204 and 206. The treated
pulp of the present invention can be dissolved in an amine solvent
by any known manner, e.g., as taught in McCorsley U.S. Pat. No.
4,246,221. The treated pulp can be wet in a nonsolvent mixture of
about 40% NMMO and 60% water. The mixture can be mixed in a double
arm sigma blade mixer and sufficient water distilled off to leave
about 12 14% based on NMMO so that a cellulose solution is formed
in block 208. Alternatively, NMMO of appropriate water content may
be used initially to obviate the need for the vacuum distillation.
This is a convenient way to prepare spinning dopes in the
laboratory where commercially available NMMO of about 40 60%
concentration can be mixed with laboratory reagent NMMO having only
about 3% water to produce a cellulose solvent having 7 15% water.
Moisture normally present in the pulp should be accounted for in
adjusting necessary water present in the solvent. Reference might
be made to articles by Chanzy, H. and A. Peguy, Journal of Polymer
Science, Polymer Physics Ed. 18:1137-1144 (1980), and Navard, P.
and J. M. Haudin, British Polymer Journal, p. 174 (Dec. 1980) for
laboratory preparation of cellulose dopes in NMMO water
solvents.
The dissolved, treated pulp (now called the dope) is forced through
extrusion orifices, called spinning, block 210, to produce latent
filaments or fibers that are later regenerated in block 212.
Finally, the lyocell fibers can be washed and/or bleached if needed
in block 214.
Owing to the compositions from which they are produced, lyocell
fibers produced in accordance with the present invention have a
hemicellulose content that is equal to or less than the
hemicellulose content of the treated pulp that was used to make the
lyocell fibers. Typically the lyocell fibers produced in accordance
with the present invention have a hemicellulose content that is
from about 0% to about 30.0% less than the hemicellulose content of
the treated pulp that was used to make the lyocell fibers. Lyocell
fibers produced in accordance with the present invention have an
average D.P. that is equal to, larger than or less than the average
D.P. of the treated pulp that was used to make the lyocell fibers.
Depending on the method that is used to form lyocell fibers, the
average D.P. of the pulp may be further reduced during fiber
formation, for example through the action of heat. Preferably the
lyocell fibers produced in accordance with the present invention
have an average D.P. that is equal to, or from about 0% to about
20% less than or greater than the average D.P. of the treated pulp
that was used to make the lyocell fibers.
The lyocell fibers of the present invention exhibit numerous
desirable properties. For example, lyocell fibers prepared from
treated pulps of the present invention comprise at least about 5
weight percent hemicellulose, cellulose having an average degree of
polymerization from about 200 to about 1100, a kappa number less
than about 2.0, and greater than 4% of fibers have a
length-weighted average fiber length of 2.0 mm. The fibers produced
according to the invention exhibit a .DELTA.R greater than or equal
to about 2.8. Preferably, such fibers have a hemicellulose content
ranging from about 5% by weight to about 27% by weight and more
preferably from about 5% by weight to about 18%, most preferably
from about 10% by weight to about 15% by weight. The average degree
of polymerization of the cellulose preferably ranges from about 300
to about 1000, more preferably from about 300 to about 1100 and
most preferably from about 400 to about 700. These fibers exhibit a
copper number of less than about 2.0, and more preferably less than
about 1.6. A lyocell fiber made according to the present invention
can have a dry elongation from about 8% to about 20%, a wet
elongation from about 10% to about 18%, a dry tenacity from about
20 to about 48 cN/tex and a wet tenacity from about 18 to about 40
cN/tex. However, the aforementioned range of properties is measured
by the proprietary assays performed by Thuringisches Institut fur
Textil-und Kunstoff Forschunge and thus, different assays may
produce different values for the same pulp. The properties of
lyocell fibers made according to the present invention can be
varied by adjusting one or plurality of conditions within the
following ranges. For example, the cellulose (i.e., pulp)
concentration in dope can be about 7% to about 30%; the spinning
temperature can be about 80.degree. C. to about 130.degree. C.; the
spinning rate can be about 40 to about 1000 meters/min., the
air-gap can be about 1 to about 50 cm; the draw ratio (defined as
winder speed/filament speed at the exit of the nozzle) can be about
1 to about 500; and anti-oxidants can be added to the dope solution
in an amount about 5% or less by weight. Suitable anti-oxidants can
include propyl-gallate or the like. However, other parameters, such
as the temperature of the dope or the spinneret can also influence
lyocell fiber properties.
Lyocell fibers of the present invention formed from dopes prepared
from treated pulp of the present invention exhibit physical
properties making them suitable for use in a number of woven and
non-woven applications. Examples of woven applications include
textiles, fabrics and the like. Non-woven applications include
filtration media and absorbent products by way of example.
Additionally, the treated pulp of the present invention can be
formed into films by means of techniques known to one of ordinary
skill in the art. An example of a technique for making a film from
the compositions of the present invention is set forth in U.S. Pat.
No. 5,401,447 to Matsui et al., and in U.S. Pat. No. 5,277,857 to
Nicholson.
The following examples merely illustrate the best mode now
contemplated for practicing the invention, but should not be
construed to limit the invention.
EXAMPLE 1
Brownstock sawdust pulp was produced in an industrial scale M&D
digester. The digester was operated at a temperature of about
182.degree. C., and average residence time in the digester was
about 60 minutes. White liquor was used as the cooking liquor in
the digester. The white liquor had a total titratable alkali (TTA)
of 115.2 grams per liter as Na.sub.2O, an active alkali (AA) of
99.2 grams per liter as Na.sub.2O, an effective alkali (EA) of 81.6
grams per liter as Na.sub.2O. Sulfidity of the white liquor was 28%
of TTA. Specific gravity of the white liquor was 1.15.
Northern Softwood sawdust unbleached alkaline Kraft pulp (main wood
species were Douglas fir, Spruce and Lodgepole pine), produced
under the stated conditions, with a kappa number of 21.0 (TAPPI
Standard T236 cm-85 and a viscosity of 110 cp (TAPPI T230) (D.P. of
1264), a copper number of 0.6 and a hemicellulose content of 14.1%
.+-.1.5% was treated with chlorine dioxide in a first D stage.
D1 Stage
The D1 stage treated pulp processed by pin fluffing the sawdust
brownstock pulp, and then transferring the pulp to a polypropylene
bag. The consistency of the pulp in the polypropylene bag was
adjusted to ten percent with the addition of water. Chlorine
dioxide corresponding to an amount equivalent to 28.4 pounds per
ton of pulp was introduced to the diluted pulp by dissolving the
chlorine dioxide in the water used to adjust the consistency of the
pulp in the bag. The bag was sealed and mixed and then held at
75.degree. C. for 1.5 hours in a water bath. The exit pH was 1.9
and no ClO.sub.2 residual was detected. The pulp D.P. was 1258. The
pulp was removed and washed with dionized water. The pulp was next
treated in an EP stage.
EP1 Stage
The washed pulp from D1 stage was then placed in a fresh
polypropylene bag and caustic was introduced with one-half of the
amount of water necessary to provide a consistency of ten percent.
Hydrogen peroxide was mixed with the other one-half of the dilution
water and added to the bag. The hydrogen peroxide charge was
equivalent to 60 pounds per ton of pulp and the caustic dose was
equivalent to 30 pounds per ton of pulp. The bag was sealed and
mixed and held for 1.5 hours at 88.degree. C. in a water bath. Only
a trace of residual hydrogen peroxide was detected. The exit pH was
8.1 and pulp D.P. was 905. After removing the pulp from the bag and
washing it with water, the mat was filtered and then placed back
into the polypropylene bag and broken up by hand. The pulp was next
treated in a second D2.
D2 Stage
Chlorine dioxide was introduced to the pulp in an amount equivalent
to 19 pounds per ton of pulp with the dilution water necessary to
provide a consistency of 10 percent. The bag was sealed and mixed,
and then held for 1.5 hours at 75.degree. C. in a water bath. The
exit pH was 2.6, and the pulp D.P. was 923. After removing the pulp
from the bag and washing it with water, the mat was filtered and
then placed back into the polypropylene bag and broken up by hand.
The pulp was next treated in a second EP stage.
EP2 Stage
The washed pulp was then placed in a fresh polypropylene bag and
caustic was introduced with one-half of the amount of water
necessary to provide a consistency of ten percent. Hydrogen
peroxide was mixed with the other one-half of the dilution water
and added to the bag. The hydrogen peroxide charge was equivalent
to 40 pounds per ton of pulp and the caustic dose was equivalent to
30 pounds per ton of pulp. The bag was sealed and mixed and held
for 1.5 hours at 88.degree. C. in a water bath. Residual hydrogen
peroxide was detected at <10 pounds per ton. Exit pH was 11.7
and pulp D.P. was 579.
The treated pulp had a copper number of about 1.4 measured by TAPPI
standard T430, a hemicellulose content of 14.1 percent .+-.1.5%,
R10 80.9%, R18 84.5% and carboxyl content of 4.0 meq/100g (28 micro
mol/gram TITK method). Final brightness of 88 ISO was achieved. The
resulting fiber analysis yielded a length-weighted average fiber
length (LWAFL) of 1.3 mm, and a coarseness of 15 mg/100 mm.
EXAMPLE 2
NMMO Monohydrate Dissolution Test
In a first dissolution test with NMMO monohydrate pulp samples
produced according to application Ser. No. 09/574,538 and the
present invention, respectively, were fluffed and mixed with
N-methylmorpholine-N-oxide monohydrate to provide two separate
mixtures. Each of the respective mixtures was then heated on a hot
plate to 85.degree. C. and the dissolution process of the fibers in
the NMMO monohydrate was observed under a light microscope of
88.times. magnification power. The microscope was equipped with a
camera. A photograph of the respective fibers was taken when the
mixture reached 85.degree. C. FIG. 3 is a micrograph of the fiber
produced according to application Ser. No. 09/574,538 in the
solvent. FIG. 5 is a micrograph of the fiber produced according to
the present invention in the solvent. Photographs were taken at
time intervals of 30 seconds. FIG. 4 shows a micrograph of the
prior application '538 fiber after a period of about 2 minutes.
FIG. 6 shows a micrograph of the fiber produced according to the
present invention after a period of about 2 minutes. As can be seen
in FIG. 6, a fiber according to the present invention was
substantially dissolved within 2 minutes, but for small vestiges of
the fiber visible in the upper right corner of the micrograph. It
should be apparent that the fiber is capable of fully dissolving in
less than 5 minutes. The pulps produced according to the present
invention are faster dissolving, capable of fully dissolving in
NMMO monohydrate within about 2 minutes.
EXAMPLE 3
Dry Jet Wet-Spun Fibers
The pulp made according to the present invention was used to
prepare a dope sample by dissolving the treated pulp in NMMO. The
dope was spun into lyocell fibers by a dry jet wet-process as
described in U.S. Pat. No. 5,417,909, which is incorporated herein
by reference. The dry jet wet-spinning procedure was conducted by
TITK. The properties of the fibers prepared by the dry jet/wet
process for the dope sample of the present invention are summarized
in Table 1. Table 2 shows a comparative lyocell fiber from a pulp
made by the method of application Ser. No. 09/574,538, also using a
dry jet wet-process. The differences between the fibers of Tables 1
and 2 reside, at least, with the raw materials used in the
process.
A second dissolution test was applied to the pulp samples used to
make the lyocell fibers of Tables 1 and 2. The pulps were
separately dissolved in NMMO at 80.degree. C. to 100.degree. C. to
yield a 0.6% solution of cellulose without minimum stirring. The
time for complete dissolution of the pulps was observed by light
microscopy at a magnification of 40.times. to 70.times.. The times
for dissolution of the pulps are set forth in Table 1 and Table 2.
The faster dissolution time of the sawdust pulp is apparent. This
is thought to be due to the generally shorter fiber lengths of the
pulp, yet the resultant lyocell fiber properties made from wood
chips and sawdust are comparable.
TABLE-US-00001 TABLE 1 Fiber Properties (Alkaline Sawdust
Pulp/Alkaline Treatment) fiber fineness (dtex) 1.42 1.60 cellulose
[pulp] content (%) 12.3 12.3 in dope hemicellulose content (%) in
14.1 14.1 washed fibers tenacity dry (cN/tex) 35.3 36.2 tenacity
wet (cN/tex) 28.7 27.9 tenacity ratio (%) 81.3 77.1 elongation dry
@ break (%) 14.0 14.6 elongation wet @ break (%) 13.2 14.2 loop
tenacity (cN/tex) 12.4 13.8 loop tenacity ratio (%) 35.1 38.1
initial modulus (cN/tex) 601 606 wet modulus (cN/tex) 185 169 fiber
DP 451 451 Dissolution time (min)* 60 60 Length-Weighted Average
1.2 1.2 Fiber Length (mm) coarseness (mg/100 m) 15 15 *Dissolution
test of Example 3.
TABLE-US-00002 TABLE 2 (Comparative) Fiber Properties (Kraft Wood
Chip Pulp/Alkaline Treatment) fiber fineness (dtex) 1.63 1.25
cellulose [pulp] content (%) 11.3 11.3 in dope hemicellulose
content (%) in 13 13 washed fibers tenacity dry (cN/tex) 40.9 42.0
tenacity wet (cN/tex) 31.0 32.5 tenacity ratio (%) 75.8 77.4
elongation dry @ break (%) 12.9 12.7 elongation wet @ break (%)
13.2 12.7 loop tenacity (cN/tex) 8.7 10.4 loop tenacity ratio (%)
21.3 24.8 initial modulus (cN/tex) 787 766 wet modulus (cN/tex) 191
213 fiber DP 462 462 Dissolution time (min)* 90 90 Length-Weighted
Average 2.05 2.05 Fiber Length (mm) coarseness (mg/100 m) 20.3 20.3
*Dissolution test of Example 3.
* * * * *